Rubber compositions for tire treads intended for use under heavy loads, such as for example truck tires, may be reinforced with precipitated silica and carbon black and may be composed of various elastomers.
However, for such tires, special considerations usually need to be made for their tire tread rubber compositions.
For example, passenger tire treads are normally used with a desired balance of relatively low rolling resistance for fuel economy, treadwear and relatively high traction for control on the road surface.
However, truck treads are normally designed for use with relatively heavy loads and the traction quality of the rubber composition is usually not as significant since the  higher loads placed on the tire itself adds to the traction of the tire tread on the road surface.
Also, significantly, such truck tire treads are usually desirably composed of rubber compositions which are designed to have less internal heat build-up to reduce the running temperature of the tire tread. Such rubber compositions often exhibit less traction between the tread and the road surface, relying more on the load in the tire to enhance the tread's traction characteristic.
Accordingly, for truck tire treads, special attention is normally placed on selection of the rubber reinforcement, including selection of carbon black reinforcement, as well as the selection of elastomers to be used with the specified reinforcement for the reduction in heat build-up quality normally desired.
It is recognized that it is well known to use various materials, and amounts of various individual materials, for tire treads such as, for example, precipitated silica, selected carbon black reinforcement, rubber processing oil, as well as individual elastomers.
However, for truck tire treads, designed to effectively carry large loads, it is considered herein that the selection of materials is more material-specific as well as more combination-of-materials specific.
In another aspect of the invention, tires with sidewalls of a specified rubber composition prepared by a prescribed order of addition of carbon black and precipitated silica is also provided.
For tire sidewalls as well as tire treads, it is recognized that cis 1,4-polybutadiene rubber has been suggested for use in their rubber compositions.
However, it is believed to be generally known that the use of relatively high levels of the cis 1,4-polybutadiene rubber in rubber compositions which also contain a relatively high concentration of carbon black reinforcement usually results in an undesirably low tear resistance (tread strength).
Tear resistance of a rubber composition is usually a very important rubber property for both tire treads and tire sidewalls.
Exemplary of suggestions for use of random high trans styrene-butadiene in various rubber compositions for various tire components, including tire treads, are, for example, U.S. Pat. No. 6,103,842.
This invention is primarily directed to a discovery, for tire tread and for tire sidewall purposes, of the use of material-specific combinations of high trans styrene-butadiene rubber, together with natural rubber or synthetic natural rubber and cis 1,4-polybutadiene rubber in combination with specific particulate reinforcements and processing oil in specified amounts, wherein the precipitated silica and carbon black are required to be provided in a prescribed order of addition. It is believed the described preparation of that such material-specific combinations, which specify defined amounts of such materials, is novel and inventive, particularly for such truck tire treads.
The rubber composition itself, depending largely upon the selection of carbon black, may also be useful as a tire sidewall or other tire components or in rubber tracks, conveyor belts or other industrial product applications.
In the description of this invention, the terms “rubber” and “elastomer” if used herein, may be used interchangeably, unless otherwise prescribed. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the term “phr” if used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight of rubber, or elastomer.”
A reference to an elastomer's Tg refers to a “glass transition temperature” which can conveniently be determined by a differential scanning calorimeter at a heating rate of 10° C. per minute.
A polymer's melting point, particularly the said high trans styrene-butadiene unvulcanized polymer, can conveniently be determined by use of a differential scanning calorimeter at a heating rate of about 10° C. per minute. Such method of melting point determination is well known to those skilled in such art.